The purpose of this proposed research is to study epithelial tumor formation in the terminal bronchioles and alveoli of the rat lung. The studies will entail exposing animals to nitrogen dioxide (NO2) and to a chemical carcinogen benzo(alpha)pyrene (BP). NO2 exposure results in tissue injury followed by proliferation of two epithelial cell types in the lung (nonciliated cells in the terminal bronchioles and Type 2 cells in the alveoli). We will introduce the carcinogen at specific times during the proliferative phase of repair to determine if it will alter this response and cause an increased incidence of tumor formation. Specically, we will study the effects of BP on (1) the duration of cell proliferation and (2) the transition of nonciliated cells to ciliated cells and of Type 2 cells to Type 1 cells. These studies results then will be related to the incidence and types of tumors formed.